One judge noted with surprise that Barack Obama "didn't look particularly happy" at being named the Nobel peace laureate. Another marvelled at how critics could be so patronising.

In a rare public defence of a process normally shrouded in secrecy, four of the Nobel peace prize jury's five judges have spoken about a selection they said was both merited and unanimous.

To those who say a Nobel is too much too soon in Obama's young presidency, "we simply disagree ... He got the prize for what he has done", said the committee chairman, Thorbjorn Jagland.

Jagland singled out Obama's efforts to heal the divide between the west and the Muslim world and to scale down a Bush-era proposal for a missile shield in Europe.

"All these things have contributed to – I wouldn't say a safer world – but a world with less tension," Jagland said.

For the nine-year Nobel committee veteran Inger-Marie Ytterhorn, Obama's demeanour spoke volumes when he first acknowledged the award during a news conference at the White House.

"I looked at his face when he was on TV and confirmed that he would receive the prize and would come to Norway, and he didn't look particularly happy," she said.

"Obama has a lot of problems internally in the United States and they seem to be increasing. Unemployment, healthcare reform: they are a problem for him."

She acknowledged there was a risk the prize might backfire on Obama by raising expectations even higher and giving ammunition to his critics. "It might hamper him," Ytterhorn said, because it could distract from domestic issues.

"[But] whenever we award the peace prize there is normally a big debate about it."

It is unusual for the Nobel jury to speak out so candidly about its selection.

Even the most seasoned Nobel watchers were surprised by Obama's prize – they hadn't expected the US president to be seriously considered until at least next year. He took office barely two weeks before the 1 February nomination deadline.

Jagland said that was never an issue for the Nobel committee, which followed the guidelines set forth by Alfred Nobel, the Swedish industrialist and inventor of dynamite who established the prize in his 1895 will.

"Alfred Nobel wrote that the prize should go to the person who has contributed most to the development of peace in the previous year," Jagland said. "Who has done more for that than Barack Obama?"

Aagot Valle, a leftwing Norwegian politician who joined the Nobel panel this year, dismissed suggestions that Obama was undeserving. "Don't you think that comments like that patronise Obama? Where do these people come from?

"I'm not afraid of a debate on the peace prize decision. That's fine."

World leaders reacted positively to Obama's Nobel in most cases, the committee said, with much of the criticism coming from the media and Obama's political rivals.

"I take note of it," said Jagland. "My response is only the judgment of the committee, which was unanimous."